It is well-known that non-speech signals, e.g. musical sound, can be more complicated in processing than human vocal sound, occupying a wider band of frequency. Recent state-of-the-art audio coding systems such as AMR-WB+ [3] and xHE-AAC [4] offer a transform coding tool for music and other generic, non-speech signals. This tool is commonly known as transform coded excitation (TCX) and is based on the principle of transmission of a linear predictive coding (LPC) residual, termed excitation, quantized and entropy coded in the frequency domain. Due to the limited order of the predictor used in the LPC stage, however, artifacts can occur in the decoded signal especially at low frequencies, where human hearing is very sensitive. To this end, a low-frequency emphasis and de-emphasis scheme was introduced in [1-3].
Said conventional adaptive low-frequency emphasis (ALFE) scheme amplifies low-frequency spectral lines prior to quantization in the encoder. In particular, low-frequency lines are grouped into bands, the energy of each band is computed, and the band with the local energy maximum is found. Based on the value and location of the energy maximum, bands below the maximum-energy band are boosted so that they are quantized more accurately in the subsequent quantization.
The low-frequency de-emphasis performed to invert the ALFE in a corresponding decoder is conceptually very similar. As done in the encoder, low-frequency bands are established and a band with maximum energy is determined. Unlike in the encoder, the bands below the energy peak are now attenuated. This procedure roughly restores the line energies of the original spectrum.
It is worth noting that in the known technology, the band-energy calculation in the encoder is performed before quantization, i.e. on the input spectrum, whereas in the decoder it is conducted on the inversely quantized lines, i.e. the decoded spectrum. Although the quantization operation can be designed such that spectral energy is preserved on average, exact energy preservation cannot be assured for individual spectral lines. Hence, the ALFE cannot be perfectly inverted. Moreover, a square-root operation is necessitated in an implementation of the conventional ALFE in both encoder and decoder. Avoiding such relatively complex operations is desirable.